Phosphate ore is sometimes treated with sufficient sulfuric acid to make phosphates available as a plant or animal nutrient without separating the calcium sulfate formed. Alternatively, in the wet acid process, acidulation with sulfuric acid can be complete to yield phosphoric acid which is separated from the calcium sulfate residue for use as a fertilizer and for other common uses of phosphoric acid.
High quality phosphoric acid is produced by the reaction of calcium phosphate raw materials with silica and carbon in an electric arc furnace to produce elemental phosphorous, which is then burned, and the phosphorous oxides formed absorbed with water to produce high-quality phosphoric acid.
The so-called wet process, in which phosphoric acid is made by contacting insoluble phosphates with dilute sulfuric acid, is very expensive because of the corrosiveness of the dilute acids used, the intricate stages and recycles necessary for proper crystal growth to facilitate the separation of the phosphoric acid from the calcium sulfate residue, and the necessity of evaporating large amounts of excess water from the phosphoric acid product to handle and ship economically.
Phosphoric acid made via the elemental phosphorous route has an even higher cost of manufacture due to the electrical energy consumed and the very high temperature furnace equipment needed, so that the product is primarily used only as a food ingredient.
A method of treating calcined phosphate ore with sulfur dioxide to convert the insoluble phosphate to increase the content of soluble phosphates is described in U.S. Pat. No. 3,544,300 which is incorporated herein by reference in entirety. In the patent the sulfur dioxide is removed from the treated ore which is used as such as a fertilizer without any step of extraction of phosphoric acid.
A method of producing phosphoric acid by treatment of phosphate ore with sulfur acid or sulfur trioxide and extraction of the phosphoric acid with water is described in U.S. Pat. No. 4,154,800 which is incorporated herein by reference.